In recent years, there has been an increased demand for high capacity communications systems and although several alternative approaches to such systems have been explored, optical communications systems now appear to be the preferred approach to meeting such demand. Optical communications systems as presently contemplated have a light source and a photodetector which are optically coupled to each other by a glass transmission line. The glass transmission line presently has a silica based composition and is commonly referred to as an optical fiber. A light source commonly used in such optical communications systems is a semiconductor laser diode. Such optical communications systems have been developed to a high degree of sophistication and are now capable of rapidly transmitting large amounts of information over long distances. In most present day systems, only one such diode is used to transmit information on any individual fiber, and the diode should ideally operate with essentially a stable single frequency spectral output. Information is transmitted as the laser emits or does not emit light pulses thus forming a bit stream, and the photodetector receives or does not receive light pulses within predetermined time intervals.
The three primary contemplated markets for lightwave systems are transmission, loop plant and local area network systems. Transmission systems typically carry many calls between central offices. Loop plant systems as used herein means those which carry calls between central offices and customer premises. Local area networks carry calls between locations that are located on customer premises, e.g., between a computer and work station, and are useful for factory or office automation. Thus, a perhaps better term is private subscriber network.
However, still greater amounts of information can be transmitted if, for example, a plurality of light sources emitting radiation at different frequencies is optically coupled into a single optical fiber at the same transmitter location. Thus, there would be a plurality of bit streams at different frequencies. Such systems have been contemplated and are commonly called wavelength division multiplexing systems. In such systems, the photodetector component includes means to individually detect the separated frequencies or wavelengths, i.e., the photodetector has demultiplexing means.
Superficially, such a system appears analogous to a radio or microwave system using a plurality of transmitters, at one location, emitting at different frequencies and a plurality of receivers at another location. One might think that radio techniques could be directly applied to optical systems without significant change. However, this is not possible. Radio circuits are typically much smaller than the radio wavelengths and microwave circuits have dimensions comparable to microwave wavelengths. Optical multiplexers smaller than an optical wavelength are not likely given the resolution of present lithography used in device fabrication and would also be difficult to couple optical energy into and out of. Optical wavelength division multiplexing devices are thus larger than optical wavelengths and fundamentally different from radio and microwave systems.
It should also be appreciated that attempts to implement narrow band optical systems face fundamental stability and resolution problems not encountered in radio. Optical frequencies are approximately 10.sup.5 times greater than microwave frequencies, and both frequency stability and resolution must be 10.sup.5 times greater in narrow band optical systems than in microwave systems. Such stability has not been possible.
The subject of wavelength division multiplexing in optical communications systems is reviewed in an article by W. J. Tomlinson in Applied Optics, 16, pp. 2180-2194, August 1977. The systems described are typical and all have a multi-transmitter module at one end and a multi-receiver module at the other end. The systems are presently contemplated only for transmission systems. They are not easily adaptable for either loop plant operation or a local area network because the optical fiber is not easily tapped at arbitrary places without first detecting all the signals and then regenerating them. Individual components discussed are, however, illustrative. Multiplexing and demultiplexing means typically comprise either gratings, prisms, or filters. While these means are perfectly adequate for many multiplexing systems, they suffer the drawback that the number of channels that the system may handle is extremely limited because the wavelength dispersion of the demultiplexing means is not adequate to separate very closely spaced channels in devices of reasonable dimensions. A relatively large channel spacing is required because the optical bandwidth must be adequate to absorb the wavelength variability due to, for example, temperature, aging, and manufacturing, of present semiconductor lasers. Also, some of the multiplexing means have a relatively high fixed loss per channel. This also limits the possible number of channels.
Other wavelength multiplexers have been described in the literature. For example, frequency selective coupling means, i.e., evanescent couplers, have been proposed as an alternative to means that rely on dispersive properties of the multiplexer components. An evanescent coupler, in its simplest embodiment, uses at least two optical waveguides in such close proximity that the propagating mode of the second waveguide is within the exponentially decaying evanescent portion of the propagating mode of the first waveguide. The overlap couples optical energy into the second waveguide if the propagation constants, k, in the two guides are equal. If the values of k are equal at only a single frequency, only energy at that frequency is coupled while energy at other frequencies remains in the first guide. H. F. Taylor describes such a frequency selective coupling scheme in Optics Communications, 8, pp. 421-425, August 1973. The couplers described used optical coupling between two nonidentical waveguides to couple the single optical frequency for which the propagation constants in the two guides are equal. Optical bandwidths of approximately several tens of Angstroms could be achieved in 1 cm long couplers thus theoretically permitting about 100 optical channels. However, these bandwidths are too narrow for use with present semiconductor lasers. There is also the problem of insuring that the multiplexer coupling frequency is accurately matched to the demultiplexer coupling frequency. However, with anticipated variations in, for example, manufacture, the narrow transmission bandwidth might not overlap the narrow reception bandwidth.
An interesting variant of this selective coupling scheme is described in Applied Optics, 17, pp. 3253-3258, Oct. 15, 1978. The system described used optical coupling between nonidentical planar waveguides to demultiplex the optical signals. While advantageously used for some applications, the coupler demultiplexers all lack a resonator component. This is disadvantageous because they need a light source having an extremely narrow spectral output.
In Fiber and Integrated Optics, 1, pp. 227-241, 1978, H. Kogelnik reviewed the subject of integrated optics. Multiplexing transmitter modules using distributed feedback semiconductor lasers are described. In one module, six distributed feedback lasers are fabricated on a single substrate and their outputs combined by a branching waveguide coupler. The manufacturing, temperature and drift variations of the lasers are identical, due to the single substrate, and a channel spacing of 20 Angstroms is possible. However, the problem of making a receiver which will match the transmitter frequencies as they drift still exists.
Other optical components suitable for use in an optical communications system, as well as other uses, have been described in the literature. For example, Applied Physics Letters, 33, pp. 940-941, Dec. 1, 1978, described a fiber gyroscope that has a fiber ring, not a Fabry-Perot, interferometer for electronic phase sensing.
In the Bell System Technical Journal, pp. 2103-2132, September 1969, E. A. J. Marcatili describes light transmission through curved optical waveguides. Of special interest from the point of view of optical communications systems is the ring Fabry-Perot interferometer depicted in his FIG. 1. However, the embodiment depicted is not well adapted for wavelength division multiplexing systems as the resonator shown picks out radiation at many frequencies without a significant amount of discrimination between the various frequencies. That is, the resonator is unable to discriminate between the multiplicity of Fabry-Perot peaks of the resonator. Another device of general interest is the integrated linear Fabry-Perot resonator described by Smith et al in Applied Physics Letters, 34, pp. 62-65, Jan. 1, 1979. The device described required no external electrical inputs and used only optical outputs. The use contemplated was in bistable optical devices.
Thus, the typical prior art wavelength division multiplexing systems use a very wide optical bandwidth to accommodate the frequency variations of semiconductor lasers and the total number of channels is very limited. Some narrower bandwidth system components were demonstrated, for example, frequency selective evanescent couplers, but have not been used in systems because of the absence of a precise narrow bandwidth source and the difficulty in matching transmitter and receiver bandwidth's center frequencies. Additionally, the systems are not easily adapted for loop plant or local area network operation.